The present invention relates to the field of optics, in particular, to optical devices for spatially separating or combining orthogonally polarized optical beams, in particular, to optical devices used as optical beam polarizers or analyzers in the optics of ultraviolet, visible, and infrared radiation, including laser emission.
According to commonly accepted rule, orientation of the light-wave electric field determines its polarization direction, and the plane of the electric vector and the light propagation direction are referred to as a polarization plane. If electric field oscillations occur only in that plane, and the plane itself has a constant spatial position, such light is referred to as having linear or planar polarization (or simply polarized). If the wave electric vector rotates around the light propagation direction (i.e., around the wave vector), such light can have either elliptical or circular polarization. For nonmonochromatic light, i.e., for one containing a number of frequency components, the temporal changes in the amplitude and spatial position of its resulting electric vector can be absolutely arbitrary, and such light is referred to as unpolarized.
Linearly polarized light beams have found general application in optics, laser engineering, technology, e.g., for precision processing of metals (cutting, drilling etc.), in photochemistry for resonance excitation of molecules and atoms, in biology for similar purposes, in communication engineering, etc. The preference is given to polarized light due to higher accuracy of interaction of such light with materials. Such high interaction accuracy results from the complexity and anisotropy in the inner structure of the aforementioned materials. For example, most of the devices widely used in optics and communication engineering for entering information into a light beam, such as electrooptical and acoustooptical modulators, operate with linearly polarized light because of the pronounced anisotropy of optical properties in the crystals these devices are based upon. Fiber optics communication engineering is a field where polarized light has a constantly increasing application. Anisotropic fibers for polarized light and low-noise polarization amplifiers have been developed. In principle, polarized radiation is used for effective transformation of laser frequencies in nonlinear crystals and for selection of optical radiation frequencies by anisotropic tunable acoustooptical and electrooptical filters. The use of polarized light is required for operation of binary polarization switchers/modulators, polarization multiplexers and, in general, in any optical devices for which anisotropic interaction of light with the materials is advantageous.
There are a number of devices that can be used for light polarization. These include dichroism dye based polarizers, purely crystalline polarizers, interference polarizers, polarizers based on isotropic materials that use the effects of light reflection and light refraction at the Brewster angle, etc. The Brewster angle in air is an angle xcfx86 under a condition tan xcfx86=N, where N is a refraction index of the optical medium. Only the light with the component of the electric vector of the light wave, which is perpendicular to the plane of incidence, is reflected, while the light with the component which lies in the plane of incidence is not reflected but refracted. The so-called Brewster law defines a ratio between a refraction index N of the optical medium and such an angle xcfx86 of incidence on this medium of a natural (non-polarized) light, at which the beam reflected from the dielectric surface is totally polarized.
However, special accent is made on prism-type polarizers that have a specific geometry and are made of optically anisotropic crystalline materials. The reason for making such accent are special properties of these polarizers. As a rule, they are crystalline polarizers that exhibit high extinction (ratio of the useful and unnecessary orthogonally polarized light components) of polarized beams, low optical losses, and high resistance to high-power optical radiation, especially laser radiation.
For better understanding the principles of the present invention, it would be advantageous to briefly describe the structure of conventional polarizing prisms. The basics of polarizing devices are described, for example, in Handbook of Optics, Vol. II, Devices Measurements and Properties, McGraw-Hill, Inc., 1995, pp. 3.1-3.70, New York, San Francisco, Montreal, Tokyo, Toronto.
Polarizing prisms are made only of birefringent crystals that have no cubic crystal symmetry. In such crystals light is split into two orthogonally polarized beams which, upon exit from the crystal, are in general case spatially separated both with respect to the exit points and the propagation angles. However, for many reasons (small separation angles or distances, unavoidable frequency dispersion of the prism, reflection optical losses and technologically uncomfortable beam exiting geometry) simple single crystal prisms are replaced for combinations thereof that are referred to as polarizing prisms. Polarizing prisms are usually made of a relatively cheap and abundant calcite (CaCO3). Recently a wide range of artificially grown birefringent crystals have been developed for polarizing prism applications. Such crystals are, for example, TiO2, YVO4, KNbO3, KTiOPO4, xcex1-BaB2O4, PbMoO4, TeO2, Te, Se, etc. However, the general use of these materials is precluded by their high cost, complexity of manufacturing compound prisms therefrom or insufficiently pronounced optical anisotropy (birefringence).
Advanced polarizing prisms usually contain two or more trihedral prisms made of optically uniaxial crystals of tetragonal, hexagonal, or trigonal symmetry having similar or different optical axis orientations and bonded to each other with transparent substances (cements) or separated from each other with a thin air or vacuum gap. Cement-free gaps are often used in prisms for short-wave radiation or high-power laser beams.
Prisms are subdivided into one-beam prisms, from which only one linearly polarized light beam exits, and two-beam prisms, that produce two light beams polarized in mutually perpendicular planes (orthogonally polarized beams). The former type prisms operate on the basis of the total internal reflection principle. A nonpolarized incident beam is split in the prism into two orthogonally polarized beams. One of these beams undergoes total internal reflection at the prism bounding and is defected, while the other beam passes through the bounding for further use or processing. Such prisms are know as the Nicol, Glazebrook, Hartnack-Prazmowsky, Ahrens, etc., prisms. FIGS. 1(a)-(f) shows some of these prisms (a), (b) and (c) are Glan-type prisms know as the Glan-Thompson (a), Lippich (b) and Frank-Ritter (c) prisms. The second row in FIGS. 1(a)-(f) shows Nicol-type prisms, i.e., the conventional Nicol prism (d), the Nicol-Halle form prism (e), and the Hartnack-Prazmowsky prism (f). The optical axes of the prisms are shown in FIGS. 1(a)-(f) with double arrows.
Variations in the structure of the prisms is normally accompanied by changes in the prisms"" names. For example, the air-gap Glan-Thompson prisms are referred to as the Glan-Foucault prisms, and the air-gap Lippich prisms as the Glan-Taylor prisms. In practice, any of these prisms can be referred to as a Glan prism. The air-gap Nicol prisms are referred to as the Foucault prisms. There also are combinations of three bound prisms, the so-called double prisms. The double Glan-Thompson prisms are referred to as the Ahrens prisms.
FIGS. 2(a)-(e) shows various types of the two-beam polarizing prisms. The optical axes of the two parts of the Rochon (a), Senarmont (b), and Wollaston (c) prisims are perpendicular to each other. The foster (d) and the beam-splitting Glan-Thompson (e) prisms have parallel optical axes. In this respect these prisms are similar to one-beam polarizing prisms, but their shape is changed so the two beams propagate in specific directions without noticeable losses.
The need for the great variety of existing polarizing prisms (not all of them are shown here) stems from the impossibility of designing a prism having universal parameters. Each polarizing prism has its individual advantages and drawbacks that determine its applicability. Prisms are characterized by a number of parameters, such as angular separation of the beams, angular aperture, extinction, spectral operation range, optical losses, resistance to high-power optical radiation and external thermal, humidity and mechanical impacts, entrance hole (geometrical aperture), linear sizes, durability, manufacturability and, of course, cost.
Example of one of the latest polarizing prism beam splitter is given in U.S. Pat. No. 6,018,048 issued on Jan. 25, 2000 to J. Pan et al. This splitter consists of a collimator and two similarly shaped birefringent crystal prisms. The light from the collimator is incident upon the first face of the first birefringent crystal prism, which also has second and third faces. In the first prism the collimated light that has passed along a normal to the first entrance face is incident onto the second face at a certain angle. The light component polarized perpendicular to the incidence plane is reflected without losses from the second face and is directed towards the third face of the first prism, while the light component polarized in the incidence plane is refracted to the second prism through the gap between the prisms. This thin gap is formed by parallel second faces of the prisms. The first (exit) face of the second prism is positioned relative to the second face of the second prism in exactly the same manner as the first face of the first prism is positioned relative to the second face of the first prism. As a result, the light that exits the second prism is refracted essentially along a normal to the first face of the second prism without cross-sectional distortions.
It is noteworthy that in all aforementioned combined polarizing prisms, including the one described in U.S. Pat. No. 6,018,418, separation of polarized beams occurs on the boundary between the two optical elements. This is important because, apart from beam splitting, optical losses occur due to fundamentally unavoidable Fresnel reflection and the cement material absorption on the boundary. As has been noted, the optical losses put limits upon the applicability of prisms in the UV range and high-power coherent laser engineering because the cement layer in the gap between the optical elements is frequently destroyed by such radiation. Vacuum and air gap prisms are used in the above applications, but in that case Fresnel losses increase due to the removal of immersion on the gap boundaries, thus the applicability of this design is limited. This problem could be solved by using very thin gaps with thicknesses on the order of wavelength, but in that case, apart from serious technological difficulties, optical losses in the reflected beam would grow unavoidably. This will occur due to the penetration/tunneling of this beam through the gap, which effect would unavoidably impair the forward beam extinction ratio. Depending on prism design, such losses may be as high as 10%.
Another disadvantage of multielement prisms is their complex and troublesome technology. In their manufacturing it is necessary to provide high optical quality on cemented surfaces, exact mutual orientation of the crystal prisms, high-quality cementing without inclusions, and uniform gap thickness. It is also necessary to take into account anisotropic thermal expansion in the prism components, especially in case of different optical axis orientations, choose an appropriate cementing composition, etc. Moreover, complex prisms usually require an increased consumption of the deficient single-crystal material, especially where all the prism elements should be single-crystals.
Many of the abovementioned problems could be solved by using simple uncemented trihedral single crystal prisms made of birefringent crystals. Beams in these anisotropic prisms, the shapes of which are similar to those of usual isotropic dispersion prisms, are split due to refraction at the prism faces. However, the direct use solely of the beam refraction does not solve the problem. As has been noted, a refracting prism splits beams into two, but the separation angles are not large due to the small difference in the refraction coefficients of most materials, except for Te, Se and TiO2. Another disadvantage is the unavoidably different frequency dispersions of the polarized beam deflection angles in case of uncomfortable skewed beam exit from the prism, which also results in an increased reflection loss. Nevertheless, in spite of their drawbacks, the aforementioned trihedral single crystal prisms still find practical application, primarily in the UV range. These prisms are usually made of weakly refracting quartz crystals (SiO2) or magnesium fluoride crystals (MgF2) which are transparent in this spectral range.
Attempts have been made to solve these problems by using the effect of polarized beam separation upon total internal reflection (hereafter referred to as TIR) in a birefringent crystal. One such attempt is a method of separating polarized beams described by B. R. Belostotski, Yu. V. Lyubarsky and V. M. Ovchinnikov in Fundamentals of Laser Engineering, Moscow, Sovetskoe Radio, 1972, pp. 125-127. According to this method, the wave vector of the extraordinary beam (the e-beam) after total internal reflection in an anisotropic crystal is oriented, in the general case, at a non-zero angle relative to the wave vector of the ordinary beam (the o-beam). It is a common practice based on the simple logic to maximize the angular separation of the beams by using such crystal orientation and providing for such beam propagation directions before and after the total internal reflection that ensures the maximum absolute difference in the birefringence refraction indices (xcex94N=|Noxe2x88x92Ne|). The applicant has found that this conception is not quite correct and that such an approach does not provide the maximum possible angle of beam separations. In the aforementioned known method the angle of separation between the beams after reflection is achieved by making the optical beam incident onto the reflecting surface at an angle of 45xc2x0 and by choosing such crystal optical axis orientations that the optical beam before the total internal reflection could propagate either along the optical axis or perpendicular thereto. Under such conditions, the o-beam will obey the rule of equal angles of incidence and reflection for the incident and reflected beams, while the e-beam will not obey this trivial rule and after reflection will propagate at a certain angle to the o-beam. Depending on the axis orientation and on whether the crystal is negative (No greater than Ne) or positive (Ne greater than No), this angle can be positive or negative.
For example, in optically uniaxial calcite (CaCO3) the angular difference between the wave vectors of the o- and e-beams after reflection for an incident optical radiation wavelength of about 1.06 xcexcm is 6.5xc2x0 inside the crystal (the radiation is incident upon the reflection surface along the optical axis) or 5.4xc2x0 (the radiation is incident upon the same surface in a direction perpendicular to the optical axis). The beams separated using that method will be further separated upon their exit to the air through the refracting exit surface. The disadvantages of this method are almost the same as for refracting prisms, i.e., small beam separation angles due to small xcex94N, uncomfortable exit of the beams, and uncompensated frequency dispersion of the e-beam reflection angle. In this method, however, the Fresnel reflection losses are slightly lower than in the case of purely refracting prisms (without additional total internal reflection). This is because the beams exit the crystal close to normal to the refracting surface.
Thus, it has been shown that the existing polarizing prisms of the type described above have a strictly limited and predetermined geometry and orientation of optical axes. In particular, in the work of B. R. Belostotski, et. al described in the aforementioned reference the incidence angles and orientations of optical axes, though provide separation of the polarized beams, are not optimized and therefore do not allow obtaining of large angles of beam separation. Furthermore, the use only of the Brewster effect or only of the total internal reflection for separation of the polarized beams does not allow obtaining a two-beam polarizer with high extinction and with low optical losses. The use of both these effects in polarizing anisotropic prisms for widening the range of polarized beam separation angle has not been known.
Furthermore, although the known combined multielement prisms ensure a wide range of angle separation, they possess a common fundamentally unavoidable drawback, i.e., existence of a thin beam-splitting gap, the negative properties of which put limits upon the applicability of the prisms and increases the cost of their manufacturing. An attempt to solve the above problem by using the aforementioned non-optimized orientations of crystals and values of angles of incidence did not allow to widen the range of separation angles.
It is an object of the present invention to provide a single-crystal anisotropic prism or a combination of prisms, at least one of which is an anizotropic one, which divide an incident beam into two orthogonally polarized beams with the increased angle of separation. It is another object to provide the prism or a prism combination of the aforementioned type which are characterized by minimal optical losses, reduced frequency dispersion of an angle between the polarized beams, and retaining the general initial propagation direction of the beams. Still another object is to provide the prisms of the aforementioned type which are free of cemented connections or air gaps between the prism elements and therefore are simple in construction and simple and inexpensive in the manufacture. It is another object of the invention to provide a method of manufacturing the prisms of the aforementioned type by widening the range of allowable angle of incidence and by selecting specific orientations of the crystal.